The present invention relates to a system which hydraulically alters and varies the preset revolutions per minute (RPM) stall range of a torque converter associated with an automatic transmission of an engine while maintaining or improving its coupling efficiency.
The torque converter is used in all automobiles that have an automatic transmission. It is physically connected to an engine""s crankshaft on one side, and both mechanically and hydraulically to the transmission on the other. The basic function of the torque converter is to effectively couple the engine""s power to the transmission and remaining drive train during each phase of a vehicle""s operation and movement. It serves two primary functions: It acts as a fluid coupling that smoothly transfers engine torque to the transmission, and it also multiplies this torque when additional performance is desirable. To make the converter an efficient driveline component, Its size and configuration must be determined to meet the requirements of each automotive application.
During operation it must first permit a certain amount of slippage (stall speed) in order to assist engine in getting into its power (torque) range. This will allow for smooth engine operation each time the vehicle starts from rest and begins to accelerate. All torque converters are manufactured with a preset RPM stall (slip) range. The proper stall setting is predetermined by a number of automotive application factors. These include the following: A) How the vehicle is used and how much it weighs, B) The amount of torque the engine produces and it""s RPM torque (power) curve, and C) The vehicle""s rear axle ratio and tire size. Changes to any of these factors will likewise affect the operation of the torque converter, and hence on the performance of the vehicle. When any of these factors are permanently changed the torque converter must be mechanically reworked to achieve the desired RPM stall speed, particularly in high performance vehicles.
Once a vehicle is in motion the initial slippage (stall speed) is nearly eliminated through the inherent fluid dynamics of the torque converter. The engine and transmission are considered to be xe2x80x9cfluid coupledxe2x80x9d at that point. Any remaining slippage is viewed as the torque converter""s coupling inefficiency. This is typical for any type of xe2x80x9cnon-lockupxe2x80x9d type converter.
When a torque converter slips during any phase of vehicle operation, the engine""s energy used to cause the slippage is converted to heat. This heat is transferred to the transmission fluid and subsequently to the transmission itself.
Converter Assembly
The torque converter is designed with three basic circular components: 1) the impeller or pump (driving component), 2) the turbine (driven or output component), and 3) the stator (reaction component). The stator allows the converter to multiply torque. Without this component the converter would be just a fluid coupling. It would only be capable of transferring an engine""s torque and not multiply it.
These components are assembled in a specific position relative to each other and enclosed in a fluid filled circular steel housing. The impeller is secured to the xe2x80x9ctransmission sidexe2x80x9d of the converter housing. The turbine is located opposite the impeller and can rotate freely before being splined to the input shaft of the transmission. The stator is positioned between each of these components. It is mounted on a one-way roller clutch that is splined to the stationary stator support that projects from the front of the transmission. Each of these components incorporates a series of radial fins or blades that permit continuous fluid flow between them. One side of the converter housing is bolted to the flex plate that rotates with the crankshaft of the engine. The other side has an open hub that is indexed into the front pump of the transmission.
Normal Torque Converter Operation
Whenever an engine is run, the converter housing rotates and spins the front pump of the transmission. This action causes the entire transmission hydraulic system to become pressurized. This includes keeping the torque converter full and pressurized (50 to 80 psi). The transmission hydraulic system also provides a continuous flow of fluid in and out of the converter, and directs the existing heated fluid to the transmission oil cooler. However, this fluid transfer and flow do not provide the necessary force to turn the transmission""s input shaft. The fluid flow through the action of the converter""s three main internal components provides the torque transfer through the converter. This fluid movement is known as xe2x80x9crotary flowxe2x80x9d and xe2x80x9cvortex flowxe2x80x9d.
Since these components form a closed unit, the fluid flow is a varying but continuous process. Rotary flow describes the fluid movement in the direction of the converter rotation around the centerline of the transmission input shaft. When the impeller and turbine components are rotating at nearly the same speed the fluid movement is considered to be at nearly 100% rotary flow and the converter at its maximum xe2x80x9ccoupling phasexe2x80x9d. Conversely, when the fluid is circulating through all three components in a spiral path and there""s great difference in rotational speed between the impeller and turbine, the fluid movement is considered to be nearly all vortex flow and the converter is at or near it""s xe2x80x9cstall phasexe2x80x9d. This flow causes the engine""s input torque to become multiplied.
Acting as a centrifugal oil pump, the impeller component initiates and maintains fluid flow as it rotates. The fluid is pumped from the impeller into the turbine. It travels in a circular motion in the direction of engine rotation. As the high velocity oil flow strikes the turbine, it""s force tries to make it rotate in the same direction. When an engine is idling, the force of this oil flow is not great enough to turn the turbine. If it were, the engine would stall with the vehicle stopped in gear. As the engine speed is increased from idle, the impeller speed also increases. Subsequently, the fluid flow and force to the turbine are also increased. This allows the turbine to transmit greater engine torque to the transmission. When the oil leaves the turbine it flows through the stator component. The stator redirects and accelerates the oil back into the impeller. This action increases both the velocity and force of the oil against the turbine fins, thereby multiplying (converting) the torque of the engine.
When a vehicle is standing still, with the transmission in gear, and the brakes applied, the torque converter is capable of multiplying engine torque by two-to-one or more when the engine""s throttle is applied. This is considered the xe2x80x9cturbine stallxe2x80x9d, or more commonly known as xe2x80x9cconverter stallxe2x80x9d. The maximum engine speed at which the turbine can be held stationary is known as the xe2x80x9crated stall RPMxe2x80x9d. The stator component is designed in such a way that it redirects the flow of oil back to the impeller in different ways depending on the speed and direction of the oil after it leaves the turbine. When the turbine is stalled, the oil leaves at a high angle striking the broad portion of the stator""s blades. This locks the one way roller clutch in the stator and prevents it from rotating. This causes an increase in the force and velocity of the oil (vortex flow) as it approaches the impeller. When the vehicle and turbine speed increases, the centrifugal force of the oil leaving the turbine also increases until it approaches the speed of the impeller. This also causes a change in the direction of oil flow leaving the turbine, so that it now strikes the back of the stator""s blades. As this occurs, the roller clutch automatically releases and the stator is able to freewheel. At this point (the converter""s coupling phase), the stator will rotate in conjunction with the speed of the impeller, turbine and oil (rotary flow). When all the torque converter""s components (including the fluid) rotate as a single unit, the torque multiplication is diminished and the converter acts as a fluid coupling.
Torque Converter Efficiency
The stator is the key component that permits the torque converter to multiply an engine""s torque, rather than merely transmitting this torque. Stator blade angles are carefully chosen for specific vehicle applications and engine configurations. This is done in attempt to maximize the effective torque multiplication without sacrificing coupling efficiency. However, even at best a given stator blade angle is a compromise. All other things being equal, the blade angle that delivers high torque multiplication and high stall speed also permits higher slippage during the converter""s coupling phase. Conversely, the blade angle that is selected for its coupling efficiency will not deliver sufficient converter stall when trying to effectively move a vehicle from a standstill.
Many attempts have been made over the years to improve this dual functionality of the torque converter. Besides improving the stator blade angle, a variable-pitch stator was developed many years ago. This permitted a high blade angle for good torque multiplication and a low angle for earlier converter coupling. Unfortunately, due to high manufacturing costs it was discontinued. The most recent attempt to make any improvement has come in the development of the xe2x80x9clock upxe2x80x9d type converter. This converter has been used in many different automotive applications. Its purpose is not only to eliminate slippage and reduce heat, but to improve fuel economy as well. The xe2x80x9clock upxe2x80x9d is normally engaged during the xe2x80x9ccoupling phasexe2x80x9d after the vehicle has achieved a constant rate of speed. It accomplishes this by utilizing either a clutch built into the torque converter or a secondary input shaft from the transmission. However, this feature is not designed to have any direct effect on the initial slippage (stall speed). That is achieved by again using a stator that delivers high torque multiplication and relying on the lock up to improve the coupling efficiency.
One of the easiest ways to alter the converter""s stall speed is to simply change its size. For a given vehicle combination as the torque converter""s diameter is decreased, the stall will increase. The principle behind this has to do with transmitted torque. The larger converters flow more fluid because the centrifugal force causes the fluid speed to increase geometrically in relation to the distance to its center line of rotation. Since the fluid velocity is higher, the force it transmits to the turbine is higher resulting in higher torque. It takes less impeller-versus-turbine rotation (slip) for a large converter to produce the same output torque as a small converter. The turbine is able to reach the impeller speed quicker and therefore the coupling phase occurs sooner. So, as the diameter of a converter is reduced, it requires more impeller-versus-turbine rotation (slip) to rotate the turbine a given amount. As a result, a smaller torque converter will permit an engine to spin higher during all phases of operation, especially at low turbine speeds.
The last area where any mechanical modifications can be made to alter a converter""s stall speed is by changing the impeller and turbine fin angles. The possibilities include changing the impeller fluid entrance or exit fin angles, or by changing the turbine fluid exit fin angle and machining the turbine fins. Of these, changing the impeller exit fin angle is the most common. Bending these fins back, opposite engine rotation reduces fluid velocity and raises the stall speed. Conversely, bending these fins toward the engine""s rotation makes the impeller a more efficient pump. The fluid velocity will be increased and deliver more torque at a lower engine rpm, lowering the converters stall speed. The problem with making these types of modifications is that they affect both the torque multiplication and coupling phases. The stall raising modification will also permit more converter slippage during the coupling phase. The improved coupling efficiency modification will reduce the stall and multiplication phase and may cause the engine to lug at lower speeds.
Variable Stall Control-Theory
A variety of changes have been made in attempts to improve a torque converter""s efficiency during each phase of its operation. they all involve making modifications to any one or all of mechanical components used when building a torque converter. Unfortunately, all the converter designs are full of compromises. Each modification attempts to alter the velocity and direction of the main motive force in the converter, namely the fluid.
In contrast to existing technologies, the xe2x80x9cVariable Stall Controlxe2x80x9d (VSC) system of the present invention uses a more direct approach to improving the overall efficiency of the torque converter. It accomplishes this by using the transmission""s hydraulic system. The VSC system momentarily displaces the initial charge of fluid to the converter and relieves its internal fluid pressure. This effectively alters both the preset operating stall range and the coupling speed and efficiency of the torque converter. Using the VSC system permits the torque converter components to be designed to maximize coupling efficiency, and yet maintain a desired stall range for any desired vehicle application. In doing so, many of the compromises built into the torque converter can be eliminated, and the overall efficiency is significantly improved.
Objects Of The Invention
It is therefore an object of the present invention to hydraulically alter and vary the present RPM stall range of a torque converter while maintaining its coupling efficiency.
It is also an object of the present invention to increase torque converter stall speed while its internal oil volume is reduced.
It is therefore also an object of the present invention to rapidly re-engage the torque converter at will when appropriate under the circumstances.
It is yet another object of the present invention to rotate the impeller at an increased speed with the same amount of engine input torque.
It is further an object of the present invention to multiply torque of a torque converter.
It is yet another object of the present invention to reduce the amount of slippage and heat generated during a torque converter""s coupling phase.
It is also an object of the present invention to improve over the disadvantages of the prior art.
Operation Of The Variable Stall Control
In keeping with these objects and others which may become apparent, the present invention takes into consideration the fact that under normal operation, as soon as the torque converter is rotated by the engine, the front pump of the transmission pressurizes the entire hydraulic system that includes the torque converter. The transmission fluid is automatically drawn from the oil pan and through the transmission""s valve body by the front pump. It is then forced through the stator support and into the torque converter. The pressure in the converter forces the exiting fluid into the cooling system and back to the transmission. This closed fluid circulation continues as long as the engine is running.
The Variable Stall Control system of the present invention is introduced into the torque converter""s lubrication line. It is set up to operate at the point the fluid reaches the valve body. When the VSC system is activated, the fluid circuit to the converter is closed, and simultaneously another fluid passage is opened to immediately relieve the internal converter fluid pressure. This action also allows the existing pressure in the converter to cause a discharge of a portion of its fluid. The fluid pressure to the cooling circuit also drops to near zero. Simply closing or restricting the fluid circuit to the torque converter (i.e. altering the fluid pressure) will not permit a discharge of fluid. Fluid discharge must occur rapidly in order to have the desired effect.
When the normal fluid circuit to the torque converter is interrupted in this way, a number of things happen which affect the converter""s stall phase. Since there is less fluid and pressure in the converter, the impeller component is now able to rotate faster with the same amount of engine input torque. Subsequently, the remaining fluid also travels faster from the impeller to the turbine. Since there is less fluid volume, it can not apply the same force required to spin the turbine as it could under normal operating conditions. This results in an immediate increase in the stall (slip) speed of the torque converter. The amount of engine torque at a specific rpm governs the amount of stall speed increase for as long as the VSC system remains active.
When the desired stall speed of the converter is reached, the VSC system is deactivated. At this point, the fluid circuit to the converter is reopened, and simultaneously the fluid relief passage is closed to immediately restore the internal fluid pressure and flow. The fluid then has sufficient force and velocity to cause the turbine to rotate. Since the fluid in the converter was already traveling at a faster velocity before deactivating the VSC system, (intensifying the function of the stator component) the recharged converter provides additional torque multiplication.
Advantages Of Variable Stall Control
The Variable Stall Control (VSC) system of the present invention has been developed to provide increased converter stall speed and torque multiplication. When this system is used in conjunction with a torque converter that is designed to provide maximum coupling efficiency, it is able to achieve its coupled phase sooner, thus reducing the amount of slippage and heat generated during both a torque converter""s stall and coupling phase.
This is of particular importance in high performance vehicles. In many such applications a xe2x80x9ctransmission brakexe2x80x9d feature is used. When xe2x80x9ctransmission brakexe2x80x9d is applied, the input shaft of the transmission is held stationary. This restricts the rotation of the turbine component in the converter during the stall phase. When the turbine is held stationary, the fluid temperature and internal pressure inside the torque converter normally increases dramatically. This is caused by the turbulence and friction of the high velocity fluid trying to impart its force onto the turbine and stator components. Because the Variable Stall Control (VSC) system reduces the fluid volume and relieves converter pressure simultaneously during the stall phase, this turbulence and friction is also momentarily reduced.